[TASK] Add basic test for ShowImageController (tx_cms_showpic)

Adds a basic test case for simulating the behavior of the eID
script `tx_cms_showpic` in the frontend rendering process.

Resolves: #100878
Releases: main, 12.4
Change-Id: I0caf70de88c57f592cd2db1716f94d243b9682f6
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79975


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>